The present invention relates to an aerofoil with an adjustable camber, and to a method of changing the camber of an aerofoil.
Many current large aero engines use a fixed Fan Outlet Guide Vane (OGV) system. The system is multifunctional in that it provides a structural support between the core and the fan case modules via the vanes themselves, which can withstand torque, fan-blade-off events and reverse thrust loads. In addition the fan OGVs have aerodynamic functions including separating the fan flow between the core and bypass duct.
FIG. 1 shows in cutaway view a part of an aero engine 10 in which air is directed rearwardly by a fan 12 and is separated into core flow 14 and bypass flow 16. A Fan Outlet Guide Vane (OGV) is shown at 18.
In order to maximise aerodynamic efficiency, whilst minimising cost, three different cambers, or profiles, of vanes are used in a cyclic pattern around the engine. These are defined as the datum camber, over-camber and under-camber.
FIG. 2 shows the different cambers of a previously considered OGV arrangement. An over-camber is represented at A, whilst B represents a datum, or normal, camber and C represents an under-camber.
As a further step to improve efficiency, the vanes each have a different cyclic stagger. This means that the angle of incidence to the fan flow is different for each vane. 3D flow modelling is used to obtain the optimum pattern.
Ideally more variation in vane camber would be utilised, but with previously considered approaches the cost would be prohibitive, as different tooling would be required to manufacture each of the differently cambered blades.
In some systems, including the one shown and described in US Patent Application No US 20050241291 variable stator vanes (VSV) are used. VSVs are a series of ganged vanes that pivot near the front of the vane. The aerodynamic profile of the vane is fixed but the effective angle of incidence can be changed. A similar arrangement can be found in US 2004240990A. Both systems use a mechanical linkage attached to an actuator in order to achieve a variation in the angle of incidence, with a fixed geometry of vane. With VSVs the form or profile of the camber is not changed. Only the angle of incidence varies.
In U.S. Pat. No. 5,433,404B an aerofoil is proposed which uses cams or jacks as actuators to change the profile of a flexible part of the surface of the aerofoil, in order to control drag and to improve shock strength.
U.S. Pat. No. 6,644,919 teaches a rotor blade that has a structural leading edge and an insert which has a flap that may be controlled by a piezo-electric actuator. The trailing edge of the structural part of the blade is flexible and may be hinged to permit the insert to be placed in position within the structural part. The actuator causes the flap to move up or down.
DE 10 2007 028 939 describes a compressor having vanes or blades formed of a piezo material which with induction of an electrical potential difference in the guide vanes and/or blades can change their geometric shape. By changing the geometric shape the aerodynamic flow around is changed so that the gas turbine engine can adapt to different operating conditions.
U.S. Pat. No. 6,465,902 teaches a controllable camber windmill blade which uses a piezoelectric actuator to adjust the camber of the blade depending on the wind conditions.